This invention relates in general to apparatus for controlling the movement of discrete articles from a supply. More specifically, it recites to a device for metering and separating electronic devices from a column supply.
In the testing and handling of integrated circuits (IC's) and other electronic devices, the IC's are usually stored in columns, whether in elongated tubes or in guides of a test-handler machine. For example, in the Model 757 test handler of Daymarc Corporation, the assignee of the present application, IC's carried in tubes are stored on an inclined conveyor table, and then picked, one tube at a time, from the table. The tube is rotated to empty into a track leading to test sites where the IC's are momentarily brought to rest, connected to a test circuit, and then sorted according to the outcome of the test. At this input track, the IC's are fed from a column of IC's held in the tube at a rate controlled by a metering wheel that is placed over the track near the output end of the tube. The wheel has a continuous elastomeric element such as an O-ring mounted at its outer periphery. The O-ring frictionally engages the IC's so that the rotation of the wheel propels the devices at a rate set by the wheel. When the wheel is stationary, it blocks the movement of IC's along the track.
This arrangement has proven to have a variety of problems. First, the O-ring wheel does not separate the IC's from one another. As a column of IC's advances under the wheel, each device is propelled along the test track at the same velocity as the preceding and following devices. There is no "singulation" of the devices by the O-ring wheel, that is, a separation of the devices from one another where the devices are initially in a close-packed, abutting relationship. Singulation is desirable (1) so that optical or other sensors can count the number of devices being input, compute the input rate, and coordinate subsequent handling operations and (2) a physical separation is in general more conducive to reliable handling and testing. With this prior art O-ring wheel, separation occurs through the action of gravity as the lead IC is released from the wheel and is on a smooth, downwardly inclined track. The main problem with this is that the O-ring wheel must operate slowly as compared to the operating speed of other test handler components. Stated in other words, the present meter wheel is a bottleneck that limits the speed of operation of the entire test handler.
Another difficulty with the O-ring meter wheel is that the material of the O-ring is temperature sensitive. For example, if the wheel is operated in a cold environment, the O-ring material is stiffer than normal and provides a less reliable grip on the IC's it contacts and propels. If the grip is sufficiently poor, the metering wheel can slip over the IC's, which, of course, reduces its value as a meter. At high temperatures deterioration of the O-ring is accelerated.
Another related consideration is that IC's being handled have varying dimensions. Larger IC's have generally thicker bodies than smaller ones (ones having fewer pins and a smaller volume). Because the IC's are guided on a fixed track, variations in the body thickness, it is necessary to adjust the position of the axis of rotation of the O-ring wheel depending on the size of IC's being tested. Finally, with the present O-ring meter wheel, besides the non-singulation and other problems delineated above, there is a need to remove static electric charges that can accumulate on the devices at or near the metering site, shortly before testing. With the present O-ring arrangement it is necessary to use a material for the O-ring which is conductive as well as resilient so that that static charge can be drained to ground.
While members such as brushes could be used in place of the O-ring, they also have deficiencies. With a brush, the bristles tend to interfere with one another to produce a continuous, non-singulating propulsion similar to that achieved with the O-ring. Stated in other words, the energy stored in a bristle as it deflects is damped by the presence and action of adjacent, closely-packed bristles. This interference and damping occurs with both wire and non-metallic bristles.
It is therefore a principal object of this invention to provide a mechanical metering apparatus that accelerates and singulates articles arrayed in a column.
Another pincipal object is to provide such a metering and singulating apparatus which can operate at higher speeds than previously possible with O-ring wheels.
A further object is to provide a metering and singulating apparatus with the foregoing advantages which can accommodate articles of varying dimensions, is highly wear reistant, and which is substantially insensitive to variations in temperature.
Yet another object is to provide an apparatus with the foregoing advantages that is mechanically simple, has a comparatively low cost of manufacture, and is highly reliable.